The convention has implemented a valve timing control apparatus for changing opening/closing timing of an intake valve and/or an exhaust valve according to an operational condition of an internal combustion engine (“engine”). For instance, there is known a mechanism for changing the opening/closing timing of the intake valve which is opened/closed in association with rotation of a cam shaft, by changing a rotational phase of the cam shaft relative to a crank shaft. Incidentally, the intake valve and the exhaust valve each has its own timing favorable for starting the engine. And, this opening/closing timing often differs from the opening/closing timing of the same valve during traveling of the vehicle. More particularly, the rotational phase of the cam shaft at the time of start of engine is often located at an intermediate position between the angle advancing side and the angle retarding side. For mechanically fixing this position as an intermediate locking phase suitable for engine start, there is known a variable valve timing mechanism having a locking mechanism for locking the rotational phase of the cam shaft at the intermediate locking phase (see. e.g. Patent Document 1). With this variable valve timing mechanism, after the engine starts at the intermediate locking phase and subsequently enters its operational state and when the hydraulic pressure builds up thereafter, the locking mechanism is released, thus allowing phase control suitable for the operational state.
Further, in order to ensure change, at the time of engine start, from the retarding phase side to the intermediate locking phase side (intermediate locking position) that is the relative phase suitable for engine start, a valve timing adjusting apparatus is known that has an (angle) advance assisting spring for assisting the phase displacement toward the advancing side (see e.g. Patent Document 2). With this valve timing adjusting apparatus, the range of urging phase of the advance assisting spring is set to the sum of the intermediate locking phase and 10 degrees, as measured from the maximal retarding phase. With this arrangement, even in the event of drop in the hydraulic pressure at the time of stop of the engine, the relative phase will have been displaced to the position beyond the intermediate locking phase, due to the urging force of the advance assisting spring, and at the time of start of the engine, by a cam reaction force, the relative phase will be displaced toward the retarding side, against the urging force of the advance assisting spring and the relative phase will be locked eventually at the intermediate locking position.
With the above-described valve opening/closing timing control technique according to Patent Document 2, at the time of stop of the engine, with utilization of the urging force of the advance assisting spring, the relative phase between the crank shaft and the cam shaft is displaced to a phase slightly beyond the intermediate locking phase; whereas, at the time of the start of the engine, the relative phase of the cam shaft is locked to the intermediate locking phase by means of the cam reaction force, and the force resulting from e.g. viscosity of oil, effective in the retarding direction, thus improving the start performance of the engine. However, once the engine has started, it is desired that the phase should be displaced slightly from the intermediate locking phase to the retarding side in order to improve emission performance and/or increase the torque at the time of low temperature. In the course of this displacement from the intermediate locking phase toward the retarding side, the urging force of the advance assisting spring acts as “resistance”, thus preventing this displacement process from proceeding smoothly.
Further, for realizing speedy displacement of the relative phase from the retarding side to the intermediate locking phase, it is desired that the urging force of the advance assisting spring be strong. However, if the cam reaction force, and the force resulting from e.g. viscosity of oil, effective in the retarding direction, are weak, the displacement from the intermediate lock position to the retarding side needs to rely on the control oil pressure in overcoming the resistive urging force of the advance assisting spring. For this reason, it is needed to set in advance the strength of the advance assisting spring such that the control operation to the retarding side is possible even with the minimal oil pressure.    Patent Document 1: Japanese Patent No. 3211713 (e.g. paragraphs 36-57)    Patent Document 2: Japanese Patent Application “Kokai” No. 2002-227621 (e.g. paragraphs 50-59).